In the food packaging industry heat sealable multi-ply films are commonly used as wrappers for foodstuffs. Confections such as chocolate candy bars are sensitive to high temperatures, and are ordinarily packaged in wrappers that employ “cold seal” adhesive sealable materials. The cold seal package sealing operation may be accomplished with sealing jaw pressure and without excessively high temperatures which can damage the foodstuff, e.g. melt the chocolate or caramel. Moreover, since these methods do not require heating the wrapper film, they can operate at higher speeds than traditional, heat seal packaging lines.
Polymeric films are generally used to create the flexible wrapper for the bar-shaped food. These films are typically biaxially oriented polypropylene films, but may be polyester, nylon, cellophane and/or polylactic acid films. Polymeric films produced with a cast or blown process can be used, but their lower machine direction tensile strength and excessive extensibility under tension or under thermal exposure may prevent adequate lamination. These biaxially-oriented polymeric films are typically used as the outer film and the inner film in a composite laminated structure.
The outer film is usually the printed web. The outer film can be surface printed with inks and a cold seal release coating or overlacquer that is on top of the inks. The outer film can also be printed on the inner surface with inks in a reverse image visible as a positive image from the outside of the clear film. The surface energy of the printable surface is typically 38 to 45 dyne/cm as measured with special dyne solutions per ASTM 2578 for sufficient wetting of the ink on the film surface. The film surface energy is dependent upon the polymer properties and also controlled with corona treatment or flame treatment. The other surface of the outer film typically has a cold seal adhesive release surface. The cold seal release surface can be a coating or an outer skin layer of a multilayer film produced with coextrusion when manufacturing the film.
The inner film has an exterior surface receptive to bonding to the cold seal adhesive. The inner film may also have a metallized coating on the opposing surface. The metallized coating layer is typically aluminum or an aluminum oxide and provides a moisture and oxygen barrier to the laminated structure. The inner film cold seal adhesive receptive surface is the surface in contact with the foodstuff or bar.
Chocolate candy bars have existed for decades with various wrappers. Most of the wrappers are a lamination of two films: an outer film laminated to an inner film to imbed the inks and thereby prevent surface abrasion. The outer film typically has a cold seal release layer on one surface and reverse printing on the other surface. The lamination method is normally adhesive lamination, a technique well known in the industry. The cold seal adhesive is applied to the inner film of the laminate of the two films via a gravure cylinder and in registration with the printed graphics or images on the outer film. The cold seal adhesive is applied in a frame pattern matching each printed impression, but on the opposite side. The registration of the cold seal lay down must align within the printed impression image on the opposite side of the web. This can be triggered by reading an eyemark printed with the graphics and on every impression. The printed impressions are typically running in both the machine direction and transverse direction of the printed web. The width of one printed impression and the width of the printed film determines how many impressions are made across the printed web width. The impression cut-off or repeat will determine the number of impressions running in the machine direction per length. This cut-off also determines the number and size of cold seal adhesive patterns etched on the circumference of the gravure cylinder.
Adhesive lamination via solvent based adhesive coating or aqueous based adhesive coating relies on controlled web tensions and oven drying. These various adhesive laminations put very little stress on the films via tensions and the oven temperatures are such that the shrinkage or expansion on the films is minimal allowing easy registration of the cold seal adhesive. The adhesive laminate is generally fed into a nip section with a pressure roller, typically made of rubber or neoprene with Rockwell hardness of 70, and an etched gravure cylinder applying the adhesive. The gravure cylinder is set to a speed in rpms to match the desired laminate web speed. To maintain cold seal adhesive registration, the laminate web speed is adjusted with extruder output, web speed and web tension.
In extrusion lamination the web is often printed to be purposely undersized by 1 mm to 3 mm depending on the film to be printed. Polyester (PET) film would have minimal undersizing whereas an oriented polypropylene (OPP) film would have greater undersizing. This is determined by the properties of the films. PET film is less extensible under thermal and mechanical stress than OPP film. Under heat exposure from the extrusion lamination in combination with web tension the laminate is purposely stretched by a few mm into registration with the cold seal adhesive cylinder running at a set speed.
The incorporation of cold seal adhesive material into a multi-ply wrapper structure has typically required the use of complex and expensive manufacturing techniques where all the plies are joined together and the cold seal adhesive is applied in register in a single process step. Known wrapper constructions employing cold seals also require relatively expensive component layers, films, and the like in order to provide the necessary sealing functions, aesthetics, and to accommodate manufacturing needs. Thus, there is a need for an improved foodstuff wrapper construction and related method of manufacture for use with heat sensitive foodstuffs such as chocolate, nutritional bars, grain and cereal bars, and other confections, and which are suitable for use in higher speed wrapping operations. Specifically, it would be advantageous to have an extrusion lamination process in tandem with the application of cold seal adhesive. Because the application of cold seal adhesive requires precise registration and extrusion lamination involves film stretching due to the molten extrudate temperature and web tension, to combine both processes in a single tandem process is challenging. Tandem process meaning the combination of two processes in a single pass. The two processes are (1) extrusion lamination of an outer film and inner film, and (2) the process of applying a cold seal adhesive to the inner film in registration to the outer film with printed graphics.